The present invention relates to the content analysis of materials. In particular, it relates to the use of microwave radiation to assist in moisture content analysis and related techniques in materials with compositions that are less responsive to the effects of microwave radiation.
The present invention relates to the content analysis of materials. Because the contents or compositions of materials give those materials their particular properties, knowing such content can be particularly important for purposes of making, using, processing, or otherwise dealing with various materials.
The techniques used to analyze the contents of various materials are almost endless. The present invention relates particularly to moisture content analysis, and has secondary, but important aspects related to the measurement of other materials in a sample, such as (but not limited to ) fats and oils, and volatiles other than water. Foods are a particularly useful example of materials for which knowledge of moisture content is quite valuable, and some cases required (e.g. many statutory and regulatory schemes apply to the content and labeling of food products). Accordingly, a number of techniques have been developed for measuring the moisture content of materials and particularly the moisture content of foods. In a conventional technique referred to as gravimetric moisture analysis, a sample is weighed, dried, and then reweighed. Comparing the two weights gives an absolute weight loss, which, if the material has not degraded, can be used to calculate and express the moisture content as a percentage.
Heat (often convection heating) is typically used to accelerate the removal of moisture from materials in gravimetric analysis techniques. The use of a typical oven or a hotplate for such purposes, however, raises a number of associated problems or difficulties. First, if heat tends to degrade the material, rather than merely drive off the moisture, the technique is unsuitable for moisture content analysis of that material. Additionally, because heated samples create air currents that can affect the output readings of sophisticated balances, heated drying techniques require relatively long waiting periods between measurements, with the measurement having to be repeated until the dried weight remains constant.
In more recent developments, microwaves have been used to drive moisture from samples and thus assist in the gravimetric analysis of such materials. This technique has worked extremely well in a number of circumstances, and appropriate moisture analyzers using microwave techniques have been available for some years, e.g., U.S. Pat. Nos. 4,438,500 and 4,457,632.
Furthermore, various improvements continue to make the use of microwave assisted techniques more feasible for more and more difficult materials. Again using food as the example, items with relatively high fat content (such as cheese and other dairy products) or low moisture content (e.g., powdered milk) can be difficult to analyze (for moisture content) using microwave techniques because they tend to cook or degrade when heated, rather than simply give up moisture.
Exemplary improvements in microwave techniques for such purposes are set forth in co-pending and commonly assigned applications Ser. No. 09/398,129 filed Sep. 17, 1999 for xe2x80x9cHigh Efficiency Cavity Design,xe2x80x9d Ser. No. 09/397,825, also filed Sep. 17, 1999 for xe2x80x9cHigh Efficiency Air Shieldxe2x80x9d; Ser. No. 09/398,130 filed Sep. 17, 1999 for xe2x80x9cMethod for Correcting Weight Measurement Errors During Microwave Heatingxe2x80x9d; and Ser. No. 09/156,086 filed Sep. 17, 1998 for xe2x80x9cMethod and Apparatus for Measuring Volatile Content.xe2x80x9d All describe improvements in methods, apparatus, and techniques for moisture and related content analysis of materials that are otherwise difficult to handle using microwave-assisted techniques. The contents of each of these pending applications are incorporated entirely herein by reference.
The most effective microwave heating of various materials tends to depend upon the presence of free moisture in the material to be heated. Thus, for samples with little or no free water, microwave heating is difficult or ineffective. In this regard a microwave apparatus incorporating a microprocessor can monitor the drying curve (weight loss v. time) of a sample and can predict the final dry weight (or moisture content) based on an initial portion of the drying curve. Such analyses usually may be conducted in about two to three minutes for samples that contain free water
Nevertheless, some or all of the moisture present in low moisture content materials, such as dried milk powder and detergents, tends to be bound water (e.g. water of hydration) or for other reasons resists removal by microwave radiation. In such cases, applying microwave radiation solely to the sample tends to be generally unsatisfactory and often unsuccessful, particularly when such materials tend to burn or degrade before all of the moisture (bound or free) has been removed.
Accordingly, techniques have been incorporated in which a sample is placed on a material that absorbs microwaves and becomes heated in response to those microwaves. U.S. Pat. No. 4,681,996 is an example of one such technique. As set forth therein, the goal is for the thermally-responsive support to conductively heat the sample to release the bound water. Theoretically, a truly synergistic effect should be obtained because the thermally heated support in turn heats the sample to remove bound water while the free water responds to, and is removed by, the direct effect of the microwaves upon it.
Additionally, when non-polar solvents are present with bound or free water in a material to be analyzed for volatiles, they are likewise volatilized by the thermal heat from the support, while free water (which may have been thermally freed from a bound form), is vaporized by the microwave radiation. Thus, volatiles are quickly removed from the sample whether the volatiles are bound water, free water, other polar materials or non-polar compounds.
The techniques set forth in the ""996 patent, however, are less successful in actual practice. As one disadvantage, the supports described therein are self-limiting in temperature response to microwaves, and thus pads of different composition are required for different desired temperatures. As another, the device and method failed to successfully address the differences in measured weight caused when air density gradients affected the internal balance. As a third disadvantage, no technique other than the self-limitation of the pad was available for measuring or controlling the temperature of the sample during the overall process, and the predictability of the pad""s temperature was erratic. As known to those familiar with content analysis, certain standardized drying tests are based on a xe2x80x9closs-on-dryingxe2x80x9d measurement in which a sample is heated to and maintained at a specified temperature for a specified time. The weight loss under such conditions provides useful and desired information, provided the test is run under the specified conditions. Thus, absent temperature control, microwave techniques cannot be used for loss-on-drying analysis.
Accordingly, it remains a goal of this industry to continue to improve the apparatus and techniques under which content analysis can be carried out using microwave-assisted techniques, and can be done so for materials that are otherwise less responsive, or even non-responsive, to microwave radiation.
Therefore, it is an object of the invention to provide an improvement in microwave-assisted techniques for content analysis.
The invention meets this object with a method of microwave-assisted content analysis. The method includes the steps of applying microwave radiation to a sample and susceptor pad while concurrently rotating the sample and pad while the microwave radiation is being applied, while concurrently weighing the sample, while measuring the temperature of the sample and pad, and while (if necessary or desired) moderating the microwaves being applied in response to the measured temperature.
In another aspect, the invention is an apparatus for microwave-assisted content analysis. The apparatus includes a source of microwave radiation, a cavity in microwave communication with said source, a balance with at least its pan (or pan equivalent) in said cavity, and means for rotating said balance pan within said cavity while concurrently measuring the weight of a sample on said pan.
In yet another aspect, the invention is a susceptor for microwave assisted content analysis that complements the method and apparatus. The susceptor is a pad formed of ablend of a first material dispersed in a second material, the first material being a material that generates heat in response to microwave radiation, and the second material being nonresponsive to microwave radiation, but conductively heated by said first material so that exposing said susceptor to microwave radiation evenly heats said pad and a sample on said pad.
These and other objects and advantages of the present invention willl become clearer when taken in conjunction with the following detailed description and the accompanying drawings in which: